In solids, molecular motion is frozen and, therefore, neither anisotropy of chemical shifts (i.e., shielding of nuclei by the surrounding electrons) nor static dipolar interactions among nuclear spins are averaged out. For this reason, it is necessary to cancel or average out them in high-resolution NMR in solids. Heretofore, dipolar interaction between heteronuclear species has been eliminated by dipolar decoupling. Anisotropy of chemical shifts has been eliminated by magic angle spinning (MAS). In the MAS method, a solid sample is tilted by 54.degree.44' (the "magic angle") with respect to the external static magnetic field and spun at a high speed of several kilohertz to average out the shield effects of the surrounding electrons. As a result, the chemical shift anisotropy is eliminated. However, where these two methods are exploited, if high abundances of homonuclear species are present in the sample, the linewidth of the spectrum is broadened by the dipolar interaction between the homonuclear species. If low abundances of homonuclear species are present such as .sup.13 C, the dipolar interaction is small. In this case, the anisotropy of the chemical shifts can be removed by rotating the sample about an axis oriented at the magic angle. However, the sensitivity is low because of the low abundances. Specifically, the natural abundance of .sup.1 H is high. For the same strength of applied static magnetic field, .sup.1 H resonates at a higher frequency and with stronger energy than other nuclear species. When rare spins such as .sup.13 C are excited and .sup.1 H is excited by irradiating a given RF field to make the power match with the power of the RF field applied to .sup.13 C, polarization is transferred from .sup.1 H to the dilute .sup.13 C, for exciting .sup.13 C. In this way, a significant sensitivity enhancement is achieved. This method is known as cross polarization (CP). Magic angle spinning and cross polarization (CPMAS method) are used in conjunction in high-resolution NMR in solids.
We now describe cross polarization in greater detail by referring to FIG. 17. First, a 90.degree. RF pulse is applied to .sup.1 H to tilt the spins of .sup.1 H toward the Y-axis, for example, as shown in FIG. 18(a). Then, the magnetization of the abundant proton spins is spin-locked with a pulse phase-shifted by 90.degree. (i.e., the produced magnetic field is parallel to the Y-axis). An RF pulse, observing pulse, is applied to .sup.13 C in synchronism with the spin-locking pulse (FIG. 17). The resonance frequency of protons .sup.1 H is given by .omega..sub.H =.gamma..sub.H B.sub.1H. The resonance frequency of .sup.13 C is given by .omega..sub.C =.gamma..sub.C B.sub.1C. The strength of the field B.sub.1H or the strength of the field B.sub.1C is controlled in such a way that the relation .omega..sub.H =.omega..sub.C holds. As a result, polarization is transferred from .sup.1 H to .sup.13 C as shown by the thick lines in FIG. 17 to thereby excite .sup.13 C. Then, as shown in FIG. 18(b), the spins of .sup.13 C are allowed to evolve within the X-Y plane (in the direction indicated by X' in the figure). The observing RF pulse is made to cease, and .sup.1 H decoupling is done at the same time. Consequently, the free induction decay signal arising from the X-Y plane component of the .sup.13 C spins is observed. The duration of the observing pulse is made different according to the kind of sample. Where .sup.13 C is coupled strongly to .sup.1 H, the duration is made short. Where .sup.13 C is weakly coupled to .sup.1 H, the duration is rendered long.
Where CPMAS method is used concurrently as described above, the observed signal is modulated, with sample spinning at a high speed. As a result, spinning sidebands of observed signal (SSB) appear. Especially, it is necessary for an instrument developing a strong RF magnetic field to rotate the sample at a higher speed, for reducing SSB. Unfortunately, if the rotational speed is increased, dipolar interaction decreases to deteriorate the efficiency of cross polarization. This in turn reduces the sensitivity (R. A. Wind, J. Magn. Reson. 79, 136 (1988)).
In an attempt to solve this problem, stop and go method (R. C. Zeigler, J. Magn. Reson. 79, 299 (1988)) and off angle method have been recently proposed. The stop and go method involves stopping the rotation only during the cross polarization, i.e., the time for which polarization is transferred. The off angle method consists in tilting the axis of rotation by an angle different from the magic angle only during the cross polarization.
Any of these methods need special mechanical contrivance. In particular, in the stop and go method, it is necessary to bring the sample from stationary condition to a high-speed rotation in a quite short time of tens of milliseconds. In the off angle method, the angle is required to be switched from off angle (90.degree.) to the magic angle in a short time of several milliseconds. In any case, the method should be applied with a very special mechanical design.
Where only normal CPMAS is adopted, when the sample is spun at a high speed, the dipolar interaction between .sup.1 H and .sup.1 H and the dipolar interaction between .sup.1 H and .sup.13 C decrease. Therefore, the amplitudes of the RF fields must be adjusted subtly. Hence, this adjustment is difficult to make. Furthermore, since proton-proton coupling becomes weaker, the ensemble of proton spins beaks into the individual spins. In this state, the RF power requirements must be satisfied for each individual spin. This makes the cross polarization condition vague.